Re: [PATCH] selftests: KVM: Handle compiler optimizations in ucall

[Andrew Jones]

On Thu, Jun 16, 2022 at 09:54:16PM +0000, David Laight wrote:
> From: oliver.upton@xxxxxxxxx
> > Sent: 16 June 2022 19:45
> 
> > 
> > June 16, 2022 11:48 AM, "David Laight" <David.Laight@xxxxxxxxxx> wrote:
> > > No wonder I was confused.
> > > It's not surprising the compiler optimises it all away.
> > >
> > > It doesn't seem right to be 'abusing' WRITE_ONCE() here.
> > > Just adding barrier() should be enough and much more descriptive.
> > 
> > I had the same thought, although I do not believe barrier() is sufficient
> > on its own. barrier_data() with a pointer to uc passed through
> > is required to keep clang from eliminating the dead store.
> 
> A barrier() (full memory clobber) ought to be stronger than
> the partial one than barrier_data() generates.
> 
> I can't quite decide whether you need a barrier() both sides
> of the 'magic write'.
> Plausibly the compiler could discard the on-stack data
> after the barrier() and before the 'magic write'.
> 
> Certainly putting the 'magic write' inside a asm block
> that has a memory clobber is a more correct solution.

Indeed, since the magic write is actually a guest MMIO write, then
it should be using writeq().

Thanks,
drew